Erythroid progenitors (erythroid colony forming cells, CFU-E; endogenous erythroid colony forming cells, eCFU-E; erythroid burst cells; BFU-E) will be enriched from bone marrow and peripheral blood of mouse, sheep, and man using lectin affinity adsorption. These studies will examine: 1) whether subsets exist within a particular class of erythroid progenitors which are separable using lectins and differ in their erythropoietin responsiveness, and 2) whether purified erythroid progenitors require other cells for normal proliferation and differentiation. In other studies fluorescein-labeled lectins and fluorescein-labeled antibody to erythropoietin will be used with the fluorescence activated cell sorter to study the surface carbohydrates and the erythropoietin binding properties of erythroid progenitors from mouse, sheep, and man. These studies will be undertaken to determine: 1) if the lectin-binding properties of erythroid progenitors are correlated with erythroid maturation and erythropoietin responsiveness, 2) if erythropoietin binds the surface of its target cells, 3) if the erythropoietin-responsiveness of an erythroid progenitor is correlated with its erythropoietin binding capacity, 4) the relative affinity of binding of erythropoietin to various erythroid progenitors, 5) species differences in surface carbohydrates and erythropoietin binding characteristics of the various erythroid progenitors, 6) if fluorescein-labeled antibody to erythropoietin can be used to purify erythroid progenitors, 7) if erythroid progenitors from patients with refractory anemia and polycythemia vera differ in their lectin and erythropoietin binding characteristics from normal erythroid progenitors. As a part of this study, preparation of monoclonal antibody to erythropoietin will be attempted.